1. Technical Field
The present invention relates generally to the field of position sensors and actuating systems. More particularly, the present invention relates to a method and apparatus for detecting errors in an actuator recentering system and for providing a fail safe solution if an error is detected.
2. Background Art and Technical Problems
Various aircraft flight control systems have significantly improved safety, precision, and efficiency of travel. Many of these improvements are due in part to control systems that automate tasks that were previously performed by the vehicle operator. Such systems typically comprise a control unit which communicates with various servo units and actuators (such as linear actuators) which in turn control the actions of the aircraft (e.g., turn, roll, climb, etc.). For example, with reference to FIG. 1, a linear actuator 10 generally includes a motor input 12, an actuator casing 14, a nut 16 fixed to a casing 14 and a threaded actuator rod 18 mounted to a structure (e.g., a flap or an aircraft frame). Actuator rod 18 is also threaded to nut 16 and passes through actuator casing 14. Additionally, various actuators typically have mounting means on a portion of actuator rod 18 for attaching to and manipulating selected components such as landing gear, flaps, and other movable aircraft parts. For example, as illustrated in FIG. 1, actuator 18 has a connector 20 which is affixed to a moveable part of an aircraft. In operation, to manipulate the moveable part, a motor/servo unit provides commutation via motor input 12 to rotate casing 14. As casing 14 rotates, nut 16 which is affixed thereto also rotates with casing 14. The rotation of nut 16 causes actuator rod 18 to linearly extend or retract (depending on the direction of rotation), thus manipulating the control surfaces, for example, flaps and rudders, and other moving parts of the aircraft. Exemplary linear actuators include those associated with various autopilot systems used to control the aircraft during various stages of a flight plan. Linear actuators such as an SM-7000 linear actuator are available from Sagem.
Further, various types of motors are used to drive the linear actuator. For example, the motors used may contain Y or Delta windings, and the motors used may include brushed or brushless DC motors. Brush motors have several undesirable aspects. For example, brush motors are typically heavy, and the brushes eventually wear down due to the constant friction between the brush and commutator. Furthermore, the brush life and the reliability of the connection between the brush and commutator are dependent on environmental factors such as temperature, humidity, dust, and other contaminates. Therefore, brushless motors are now typical in the aviation industry.
The motors are often controlled by various autopilot and flight control systems which control the aircraft. However, given that controlling an aircraft generally calls for smooth control, it is often desirable that the control systems be fail passive. Stated otherwise, should the control systems or any components of the control systems fail, the aircraft""s controls should not act in an undesirable or catastrophic manner (such as a sudden turn, roll or dive). For example, under various failure conditions, existing control systems have the potential to cause components, such as linear actuators, to fully drive the component in one direction or another. This is often referred to as a xe2x80x9crunawayxe2x80x9d condition.
A runaway condition might occur, for example, in a linear actuator recentering system. A linear actuator recentering system is generally designed to be momentarily accessed upon disengagement (either normal or abnormal disengagement) of the auto pilot system. The recentering system is momentarily accessed for the purpose of recentering the actuators to cause a smooth transition from auto pilot to manual control. Upon disengagement, the recentering system drives the actuators to a center position and then sets a brake on the actuator. However, if an error exists in the sensors that monitor the position of the actuator, or if some other error or failure in an actuator recentering component causes the recentering system to command the actuator away from center, it is possible that the centering system could fail and cause a runaway condition. Furthermore, the failure might only affect the actuator recentering function upon disengagement from the automation. It is therefore possible for a component to fail and for the aircraft to continue to fly for hours without any adverse consequences. Because the recentering system is not active during normal operation it is possible for this failure to remain latent (undetected) and manifest itself only upon disengagement. Upon disengagement, the aircraft may perform unpredictably if the actuator xe2x80x9cruns away.xe2x80x9d
A runaway actuator could occur for example if a binary position indicator failed or if the control electronics failed. In this example, a signal might be sent indicating that the actuator was retracted from the neutral position when the actuator was actually extended from the neutral position. Unchecked, this signal can cause the recentering function to fully extend the actuator in the recentering function""s attempt to return an extended actuator to a neutral position. In this situation, the actuator may be driven to full extension, i.e., drive the actuator xe2x80x9chard overxe2x80x9d. Furthermore, the actuator brake may not prevent the runaway condition because the actuator brake generally has a built in delay allowing the recentering.
Furthermore, other attempts to improve the reliability of the actuator recentering system generally focus on redundant systems. These redundant systems have two or more sensors as well as redundant control electronics. Solutions utilizing two of each component in the system are expensive, and in the aggregate add undesirable weight to the aircraft.
Accordingly, methods, apparatus and systems are therefore needed in order to overcome these and other limitations of the prior art. Specifically, there is a need for an independent motion direction sensor and associated logic enabling continuous monitoring of a recentering function, thereby guarding against failures of recentering controls, and thus improving the safety of auto pilot actuators and aircraft controls. Furthermore, a need exists for improved reliability recentering devices which do not require fully redundant equipment and systems.
The present invention provides methods, systems, and apparati for fail passive error detection and control of a linear actuator recentering system. In an exemplary aspect of the present invention, linear actuator controls are designed to be fail passive. In one exemplary embodiment of the present invention, a recentering device is provided which receives a signal from an independent motion direction sensor and enables continuous monitoring of a recentering function, thereby detecting recentering control failures. In another exemplary embodiment, the present invention comprises a recentering motor control commutation device, an actuator movement direction sensor and an inverse commutation device for determining a centering polarity. The centering polarity is further provided to an aircraft control system which suitably provides continuous verification of the recentering commutation by comparison to linear actuator position determined by another sensor. In accordance with another exemplary embodiment, after detecting the failure of a control component, a brake is immediately set.